The invention relates to a circuit breaker, in particular to a residual-current circuit breaker or residual-current device, having a variable time delay.
Such a circuit breaker is used to ensure protection against a dangerous body current in an electrical system. Such a dangerous body current occurs, for example, when someone touches a live part of an electrical system. The fault current or residual current then flows to ground via the person, as a body current. The circuit breaker which is used for protection against dangerous body currents disconnects the relevant circuits from the power supply system safely and quickly when the so-called rated fault current is exceeded.
The construction of a circuit breaker is known, for example, from xe2x80x9cetzxe2x80x9d (1986), Issue 20, pages 938 to 945. Outline circuit diagrams and functional principles of a fault-current circuit breaker (FI circuit breaker) and a residual-current circuit breaker (RC circuit breaker) are illustrated there, in particular in FIGS. 1 to 3.
Fault-current circuit breakers and residual-current circuit breakers are constructed in a similar way from three assemblies. When a fault current occurs, a voltage signal is induced in the secondary winding of a core-balance transformer, through whose transformer core all the current-carrying conductors of a conductor network are passed, and this voltage signal actuates a release which is connected to the secondary winding via tripping circuit electronics. For its part, the release is coupled to a switching mechanism, via which the contacts of a power breaker connected in the line or in each line are opened when the release responds. In this case, the fault-current circuit breaker takes the energy required for tripping from the fault current itself, independently of the power supply system voltage, while, in a residual-current circuit breaker, tripping is carried out in a manner dependent on the power supply system voltage. To this end, the signal emitted from the core-balance transformer is supplied, when a fault current occurs, to the residual-current tripping circuit of the residual-current circuit breaker, or of the residual-current device, with this tripping circuit being supplied from the line network.
Such residual-current circuit breakers are often provided with a tripping time delay, which does not enable the actuation of the release when a fault current occurs until after a variable delay time. This prevents a brief fault current, arising from normal operation, leading to undesirable tripping of the circuit breaker, and thus the interference with the operating processes as a result of power supply system disconnection. Both the boundary conditions in which tripping of the circuit breaker must take place and the boundary conditions in which such a circuit breaker must not trip are specified, for example for residual-current devices in power breakers, in IEC Standard 947-2. Compliance with the said Standard is tested, on the one hand, by means of a no-trip test. In this case, a sinusoidal fault current of magnitude 2xc3x97Ixcex94n (Ixcex94n=rated fault current) and with a length equal to the delay time set is produced. In this case, the circuit breaker being tested must not trip. On the other hand, a tripping test is specified in IEC 947-2, during which the circuit breaker must trip within a limited time when a fault current of a defined magnitude occurs.
Simultaneous satisfaction of both tests and additional satisfaction of the surge-current test, which is likewise required, and in which no-trip is required in response to very brief (for example, 8 xcexcs rise time and 20 xcexcs fall time to half the peak value) fault currents whose amplitude is nevertheless very high (for example 250 A), demands a specific solution, particularly when the aim is to achieve a time delay which is as constant as possible, irrespective of the type of fault current, with designs which are sensitive to pulsed current or all currents and have to satisfy various tripping requirements for different tripping fault currents.
A circuit breaker according to the precharacterizing clause of claim 1 is known from PATENT ABSTRACTS OF JAPAN vol. 018, No. 434 (E-1592), Aug. 12, 1994 (1994-08-12) and, JP 06 133448 A (MATSUSHITA ELECTRIC WORKS LTD), May 13, 1994 (1994-05-13). The application there is ground-fault detection.
The invention is thus based on the object of specifying a delay circuit for a residual-current circuit breaker having a variable tripping time delay, which delay circuit has a high withstand current and a tripping time delay which is as constant as possible and is independent of the type of fault current.
According to the invention, this object is achieved by the features of claim 1. According to this, the delay circuit comprises a smoothing module which passes on an input voltage, derived from a fault current, in smoothed form to two series-connected comparators, between which a time delay module with a variable delay time is located.
The two comparators and the time delay module connected between them mean that any fault current which occurs does not lead to the circuit breaker tripping unless the associated input voltage exceeds a predetermined switch-on level at least for a time period corresponding to the delay time set. The smoothing module prevents the tripping time delay from being considerably lengthened by the comparator switching threshold being continually overshot and undershot as a result of the signal ripple, which is dependent on the type of fault current, in the case of fault currents with a high signal ripple level, for example in the case of phase-gated fault currents, and thus prevents the tripping time delay from acting in a different way as a function of the type of fault current.
There is preferably a surge-current filter upstream of the first comparator. This prevents short-term surge currents with a low repetition rate from causing the circuit breaker to trip inadvertently when the delay time set is short.
The smoothing module is expediently in the form of a diode with a downstream low-pass filter in the form of an R-C element, whose capacitor is discharged via a first discharge circuit provided with a non-reactive resistor. This makes it possible to preset the charging time constant and the discharge time constant of the low-pass filter independently of one another. This in turn allows a desired smoothing level to be set particularly precisely. In this case, it has been found to be particularly advantageous for the discharge time constant to be selected to be longer than the charging time constant for example by a factor of between 5 and 15.
In order to prevent the first low-pass filter from unnecessarily lengthening the integration time constant (which is required to satisfy the no-trip test) of the time-delay module owing to its discharge time, and thus making it more difficult or impossible to satisfy the tripping time limit values required in this case, a zener diode is connected in parallel with the first discharge circuit. The maximum voltage applied to the capacitor is limited by this zener diode, so that said capacitor in turn discharges quickly. The breakdown voltage of the zener diode is advantageously only slightly greater than the switch-on level of the first comparator. This effectively prevents the capacitor from being overcharged above the value required to trip the switching process of the first comparator.
This shortens the time period during which the capacitor voltage is below the reference voltage once again after interruption of the fault current.
The conventional circuitry of the first comparator, with two non-reactive resistors, advantageously provides the first comparator with switching hysteresis. Switching hysteresis provides two threshold voltages, rather than a single threshold voltage, as the switch-on level and switch-off level. In this case, the comparator is switched on when the higher threshold voltage, which is used as the switch-on level, is exceeded. The process of switching off the comparator occurs only when the lower threshold voltage, which is provided as the switch-off level, is undershot. This prevents the signal ripple on the input voltage leading to the comparator switching on and off repeatedly when the input voltage is close to the switch-on level. The switching hysteresis, and thus the difference between the switch-on level and the switch-off level, are expediently matched to the parameters of the smoothing module. In this case, the switching hysteresis is preset at a lower level, the greater the smoothing level. Experiments have shown a switching hysteresis of xe2x89xa6100 mV to be particularly advantageous.
The time-delay module is implemented, in a simple manner, in the form of a second capacitive low-pass filter. In this case, the capacitance is varied in order to set the delay time. In one particularly advantageous embodiment, a number of capacitors with different capacitances are provided for this purpose, of which in each case one orxe2x80x94as an equivalent to a capacitor having zero capacitance-none is connected as the capacitance in the low-pass filter. In this way, the delay time can be varied with a number of steps.
The surge-current filter is preferably in the form of a third low-pass filter, which is decoupled by a diode and is in the form of an R-C element with a second discharge circuit. In this case, the discharge time constant is advantageously more than 50 times the charging time constant. Owing to the comparatively long discharge time, the surge-current filter acts only on brief surge currents with a low repetition rate. The zener diode connected in parallel with the first discharge circuit at the same time provides voltage limiting for the second discharge circuit.
The advantages achieved by the invention are, in particular, that, by means of a delay circuit having two series-connected comparators between which a time-delay module is located, and having a smoothing module upstream of the first comparator, the circuit breaker has a no-trip response, irrespective of the type of fault current, to brief fault currents.
The preferred interconnection of three low-pass filters whose charging time constant and discharge time constant ratios differ on the one hand effectively prevents the tripping of the circuit breaker under the influence of a brief surge fault current, irrespective of the delay time set. On the other hand, when a fault current which is present all the time occurs, it allows a tripping time which is not unnecessarily long, and thus tripping-time limiting in compliance with the regulations.